Bipolar zero-gap type electrolytic cell

ABSTRACT

A bipolar zero-gap electrolytic cell comprising an anode comprising an anode substrate constituted of a titanium expanded metal or titanium metal net of 25 to 70% opening ratio, which anode after coating the substrate with a catalyst has a surface of 5 to 50 μm unevenness difference maximum and has a thickness of 0.7 to 2.0 mm. In this electrolytic cell, the possibility of breakage of ion exchange membrane is low, and the anolyte and catholyte have a concentration distribution falling within given range. With this electrolytic cell, stable electrolysis can be performed for a prolonged period of time with less variation of cell internal pressure.

TECHNICAL FIELD

The present invention relates to a bipolar, zero-gap type electrolyticcell.

This is a bipolar electrolytic cell for use in a filter press typeelectrolyzer. The electrolyzer has many bipolar electrolytic cellsarranged though the intermediary of cationic exchange membranes, each ofwhich comprises an anode chamber and a cathode chamber arranged back toback. In the cathode chamber, there are at least two layers of aconductive cushion mat layer and a hydrogen generating cathode stackedover the cushion mat layer in an area where it contacts the cationicexchange membrane.

This electrolytic cell has an anode having a base material formed of atitanium expanded metal or titanium wire mesh with an open-areapercentage of 25% to 70%. The surface of the anode, after the basematerial has been applied with a catalyst, has a maximum heightdifference of 5 μm to 50 μm between ridges and troughs. The anode is 0.7mm to 2.0 mm thick.

BACKGROUND ART

Many proposals have been made on an ion exchange membrane type alkalichloride electrolytic cell for producing highly pure, alkaline metalhydroxides with a high current efficiency and a low voltage. Among themthere are proposals concerning a zero-gap type in which an anode and acathode are in contact with each other with an ion exchange membraneinterposed therebetween.

U.S. Pat. No. 4,444,632, JP-B-6-70276 (corresponding to U.S. Pat. No.4,615,775 and European Patent No. 124125) and JP-A-57-98682(corresponding to JP-B-1-25836, U.S. Pat. No. 4,381,979 and EuropeanPatent No. 50373) have proposed electrolytic cells using wire mats.Japanese Patent No. 2876427 (corresponding to U.S. Pat. No. 5,599,430)has proposed a mattress for an electrochemical bath.

Some of these patents have an expanded pressure plate and a cathode finemesh screen. In these electrolytic cells, however, the mat strength,anode shape, electrolyte concentration distribution or in-cell pressurevariations are not appropriate, which in turn gives rise to problems ofan undesirable increase in voltage and breakage of the ion exchangemembrane.

JP-B-5-34434, JP-A-2000-178781, JP-A-2000-178782, JP-A-2001-64792,JP-A-2001-152380 and JP-A-2001-262387 disclose elastic mats and thestrength thereof. These references also disclose the strength ofcathodes and a way to prevent collapse of the mats.

These improvements are effective. However, at a high current density ofmore than 5 kA/m², the improvements are not enough for electrolysis witha stable long-term current efficiency and voltage.

Other zero-gap electrolytic cells use springs. For example,JP-A-10-53887 discloses an electrolyzer using a spring. However, thespring increases pressure in local areas and may cause damages to amembrane in contact with it. Electrolyzers that can employ the zero-gapstructure are shown in, for instance, JP-A-51-43377, JP-A-62-96688 andJP-A-61-500669 (corresponding to WO85/2419).

These unit electrolytic cells have no air-liquid separation chamberformed within them and extract gas and liquid upwardly as is in anair-liquid mixed phase. This causes vibrations in the unit electrolyticcells and gives rise to a problem of possible breakage of the ionexchange membrane. Further, they have no provisions inside for mixingelectrolyte and have a problem that a large volume of electrolyte has tobe circulated to evenly distribute the electrolyte within theelectrolytic chamber.

JP-A-61-19789 and JP-A-63-11686 disclose a way to extract gas andelectrolyte downwardly rather than upwardly. However, gas and liquid mayin some cases be drawn out in a mixed phase, making it impossible toprevent vibrations inside unit electrolytic cells. Further, a conductivedispersion member or current distribution member intended for internalcirculation of the electrolyte is provided to make electrolyteconcentration uniform in the cells, but this has a drawback of makingthe electrolyte cell structure complex.

JP-U-59-153376 discloses a wave elimination plate as a countermeasurefor preventing vibrations in an electrolytic cell. This alone, however,can not provide enough wave elimination effect, and it is impossible tocompletely eliminate vibrations caused by pressure variations in theelectrolytic cell.

JP-A-4-289184 and JP-A-8-100286 disclose a cylindrical duct and adowncomer for internally circulating an electrolyte to make theelectrolyte concentration uniform in the cells. This, however, makes thestructure in the electrolytic cells complex and increases themanufacturing costs. Further, for electrolysis at a high current densityof more than 5 kA/m², the electrolyte concentration distribution isstill large enough to have possible adverse effects on the ion exchangemembrane.

Furthermore, although these publications attempt to prevent vibrationsby (1) providing an air-liquid separation chamber having a relativelylarge volume and by (2) extracting gas and liquid in a separated statedownwardly or horizontally, vibrations may still occur in some cases ata high current density of more than 5 kA/m².

DISCLOSURE OF INVENTION

The invention has an object of providing a bipolar zero-gap typeelectrolytic cell and an electrolysis method that enable stableelectrolysis at a high current density with a simple and reliablestructure.

More specifically, the object of the invention is to provide a bipolarzero-gap type electrolytic cell, which has a zero-gap structure with asturdy ion exchange membrane that rarely breaks, in which anode liquidand cathode liquid have a predetermined range of concentrationdistribution. It is a goal to allow electrolysis with decreased in-cellpressure variations and therefore increased long-term stability whenperforming electrolysis at a high current density of more than 4 kA/m²with use of a zero-gap ion exchange membrane type electrolyzer. It is afurther goal to provide an electrolysis method for the cell.

Another object of the invention is to provide a bipolar zero-gap typeelectrolytic cell that enables electrolysis with long-term stability bypreventing possible damage of an ion exchange membrane caused by gasvibrations in the electrolytic cell.

This invention provides a bipolar, zero-gap type electrolytic cell whichelectrolyzes an alkali chloride water solution by using a cationic ionexchange membrane. More specifically, the bipolar, zero-gap typeelectrolytic cell is intended for use in a filter press typeelectrolyzer which has a plurality of bipolar electrolytic cells and aplurality of cationic exchange membranes each disposed between theadjoining bipolar electrolytic cells.

This cell is characterized by an anode chamber, an anode installed inthe anode chamber, a cathode chamber arranged back to back with theanode chamber, and a cathode having at least two stacked layers in thecathode chamber. The anode is formed of an anode base material includinga titanium expanded metal or titanium wire net with an openingpercentage of 25-75%. After a catalyst is applied to the anode basematerial, the anode has a maximum height difference of 5-50 μm betweenits surface irregularities and a thickness of 0.7-2.0 mm. The layers ofthe cathode include a conductive cushion mat layer and a hydrogengenerating cathode layer. The hydrogen generating cathode layer adjoinsthe cushion mat layer and is arranged in an area where it contacts thecationic exchange membrane.

This construction maintains an appropriate zero-gap between the anode,the cationic exchange membrane and the cathode, allows generated gas topass through, and thereby makes it possible to minimize damage to theion exchange membrane and in-cell pressure variations and carry outstable electrolysis for a long term.

The anode base material includes the titanium expanded metal, which ispreferably formed by expanding a titanium plate and then roll-pressingit. The thickness of the expanded metal is preferably set to 95-105% ofits thickness before expansion by the roll-pressing.

The hydrogen generating cathode is formed of a base material which has athickness of 0.05-0.5 mm and is chosen from a group of a nickel wirenet, a nickel expanded metal and a stamped, porous nickel plate. Thehydrogen generating cathode preferably has an electrolysis catalystcoating layer which is formed on the hydrogen generating cathode and hasa thickness of 50 μm or less.

With this construction it is possible to easily manufacture theelectrodes at a low cost, which have appropriate flexibility andtherefore hardly damage the ion exchange membrane.

The electrolytic cell may include gas-liquid separation chambers formedintegrally with non-current-carrying portions at tops of the anodechamber and cathode chamber. In this case, at least one of a cylindricalduct and a baffle plate that serve as an internal circulation path forelectrolyte is preferably provided between a separation wall of at leastone of the anode and cathode chambers and the associated electrode.

The gas-liquid separation chambers are preferably formed with separationplates.

The gas-liquid separation chambers are installed by extracting generatedgas from the tops of the electrode chambers, thereby preventing gasvibrations and allowing more stable electrolysis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing an example of a cathode that may be usedin the bipolar, zero-gap type electrolytic cell of the invention.

FIG. 2 is a perspective view showing a L-shaped portion in an example ofa conductive plate applicable to the invention.

FIG. 3 is a plan view showing an example of an anode that may be used inthe bipolar, zero-gap type electrolytic cell of the invention, andshowing sampling positions of electrolyte concentration.

FIG. 4 is a sectional side view showing an example of an anode chamberthat may be used in the bipolar, zero-gap type electrolytic cell of theinvention.

FIG. 5 is a sectional side view showing an anode side gas-liquidseparation chamber that may be used in the bipolar, zero-gap typeelectrolytic cell of the invention.

FIG. 6 is a sectional view of the bipolar, zero-gap type electrolyticcell according to an embodiment of the invention.

FIG. 7 is a partly cutaway assembly drawing showing an application ofthe electrolyzer using the cell of the invention, in which an ionexchange membrane 28, the anode chamber and a cathode chamber are fixedwith a cathode gasket 27 and an anode gasket 28 respectively interposedtherebetween.

FIG. 8 is a plan view showing an example of a cathode that may be usedin the bipolar, zero-gap type electrolytic cell of the invention, andshowing sampling positions of electrolyte concentration.

FIG. 9 is a sectional view showing the bipolar, finite-gap typeelectrolytic cell according to another embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Generally, what are required for performing stable electrolysis ofalkali chlorides and producing chlorine, hydrogen and caustic soda at alow cost are a low facility cost, an ability to electrolyze at lowvoltage, an ion exchange membrane hardly damaged by vibrations in a cellor like, uniform distribution of electrolyte concentration in the celland stability of ion exchange membrane voltage and current efficiency,etc.

To meet these requirements, alkali chloride electrolysis based on an ionexchange membrane method has remarkably been improved in performance inrecent years. Particularly notable are the performance improvements ofion exchange membranes, electrodes and unit electrolytic cells. Forexample, an electric power consumption rate for the ion exchangemembrane method has decreased to 2000 kW/NaOH-t for 4 kA/m² or less inrecent years, down from the 3000 kW/NaOH-t that was required when theion exchange membrane process became available.

However, with the recent increased demands for large facilities, reducedenergy consumption and higher efficiency, it is desired not only toenable electrolysis at an electrolysis current density of 4-8 kA/m² evenin the electrolytic cells up from the initial 3 kA/m², but also tominimize a cell voltage.

Under these circumstances, the present inventors have consideredimproving unit electrolytic cells in an effort to realize stableelectrolysis that can be performed by using a high current density of4-8 kA/m² at a significantly lower voltage than that of the conventionalelectrolytic cells.

Ordinarily, a cationic exchange membrane is pressed against the anode bya pressure from the cathode chamber, and there is a gap formed betweenthe cathode and the cationic exchange membrane. In this gap a largenumber of bubbles as well as electrolyte exist and therefore itselectric resistance is very high. For significantly reducingelectrolytic cell voltage, it is most effective to make a distancebetween the anode and the cathode (hereinafter referred to as anelectrode distance) as small as possible to eliminate influences of theelectrolyte and gas bubbles present between the anode and the cathode.

Conventionally, the electrode distance is normally 1-3 mm (hereinafterreferred to as a finite gap). Some means have already been proposed tominimize the electrode distance.

However, the electrolytic cells generally have a conduction area of morethan 2 m², and it is impossible to make the anode and the cathodecompletely flat and smooth and set the tolerance of manufacturingprecision to almost zero mm. Therefore, simply reducing the electrodedistance cannot achieve an ideal zero-gap state, since the ion exchangemembrane installed between the anode and the cathode is broken bypressing and cutting, or since the electrode distance is almost equal tothe thickness of the ion exchange membrane and there are portionsbetween the anode and the membrane and between the cathode and themembrane, which can not be kept in a an almost no gap state (hereinafterreferred to as zero-gap).

In the ion exchange membrane, the anode has a construction of relativelyhigh rigidity to reduce deformation even when being pressed by the ionexchange membrane, and only the cathode side is made of a flexibleconstruction for absorbing irregularities caused by the manufacturingprecision tolerance of electrolytic cells, the deformations ofelectrodes and so forth to thereby keep the zero-gap state.

The zero-gap structure is required to have at least two stacked layersof a conductive cushion mat on the cathode side and a hydrogengenerating cathode adjoining the cushion mat and placed in an area thatcontacts the cationic exchange membrane. For example, it preferably hasat least three layers, as shown in FIG. 1, in which a conductive plate 3is installed in the cathode chamber, a conductive cushion mat 2 isstacked on the conductive plate, and a hydrogen generating cathode 1having a thickness of 0.5 mm or less is stacked on the conductivecushion mat in an area where it contacts with the cationic exchangemembrane.

The conductive plate 3 serves to transmit electricity to the cushion mat2 and the hydrogen generating cathode 1, both stacked over theconductive plate 3, to support the weight of these members and to passthe gas generated from the cathode toward a separation wall 5 sidesmoothly. Thus, the conductive plate is preferably formed of suchmaterials as expanded metal and stamped porous plate. An openingpercentage is preferably more than 40% to allow the hydrogen gasgenerated from the cathode to be extracted toward the separation wallside. As for the strength, when the interval between ribs 4 is 100 mm,the conductive plate can perform its function, if a pressure of 3 m-H₂Ois applied to the center of the plate, as long as its deflection is lessthan 0.5 mm. As for the material, nickel, nickel alloy, stainless steeland iron may be used from the standpoint of corrosion resistance. Interms of conductivity, nickel is most preferable.

The conductive plate 3 may be formed with an L-shaped portion 6, asshown in FIG. 2, and be directly attached to the separation wall 5. Inthat case, the L-shaped portion serves both as the rib and theconductive plate and advantageously allows saving of material andreduction of the assembly time.

For the conductive plate, it is possible to use the cathode as is, whichhas been used in the finite gap electrolytic cell.

The cushion mat has to rest between the conductive plate and thehydrogen generating cathode and transmit electricity to the cathode andto smoothly pass the hydrogen gas generated from the cathode to theconductive plate side. The most important role is to apply to thecathode in contact with the ion exchange membrane uniform pressure at alevel that will not damage the membrane in order to keep the cathode inintimate contact with the ion exchange membrane.

As for the cushion mat, a commonly known cushion mat may be used. A wirediameter of 0.05-0.25 mm is preferably used for the cushion mat. If thewire diameter is less than 0.05 mm, the cushion mat may easily collapse.If the wire diameter is larger than 0.25 mm, the cushion mat becomesstrong and, when used for electrolysis, this may adversely affect theperformance of the membrane because of the increased pressing force.

More preferably, a wire diameter in a range of 0.08-0.15 mm may be used.For example, nickel wires of about 0.1 mm diameter may be woven and thencorrugated. As for the material, nickel is ordinarily used because ofits high conductivity. A thickness of 3-15 mm may be used for thecushion mat.

Even more preferably, a thickness of 5-10 mm may be used. Flexibility ofthe cushion mat may be in the known range. For example, the flexibilityof the cushion mat may be such that a repulsive force when the mat iscompressed by 50% is in the range of 20-400 g/cm². Repulsive forcesmaller than 20 g/cm² during the 50% compression is not preferable,since it cannot completely press the membrane, and a repulsive forcegreater than 400 g/cm² is also not preferred, since it presses themembrane too strongly.

More preferably, it is possible to use a mat having an elasticity suchthat the repulsive force during the 50% compressive deformation is30-200 g/cm².

Such a cushion mat is stacked on the conductive plate for operation.Commonly known methods may be used for this installation, for instancethe cushion mat is fixed by spot welding or by resin pins or metalwires.

The cathode may be stacked directly on the cushion mat. Alternatively,it may be stacked through a separate conductive sheet. Preferably, thecathode used in the zero-gap structure has a small wire diameter and asmall number of meshes because such a cathode has good flexibility. Thecathode may be formed of a commonly available base material having awire diameter of 0.1-0.5 mm and sieve opening of 20-80 meshes.

For the base material of the cathode, it is also preferable to use anickel expanded metal, a stamped nickel porous metal and a nickel wirenet, which have a thickness of 0.05-0.5 mm and an opening percentage of20-70%.

Considering handling of the cathode during the manufacturing process andits flexibility, it is preferable to use a nickel expanded metal, anickel stamped porous plate or a nickel wire net with a thickness of0.1-0.2 mm and an opening percentage of 25-65%. In the case of thenickel expanded metal, it is preferable to roll the expanded metal toflatten it to a thickness range of 95-105% of the thickness beforeflattening. In the case of the wire net, two lines cross each other at aright angle, and the plate thickness is two times the wire diameter. Itis also preferable to roll the wire net in a thickness range of 95-105%of the wire diameter.

The cathode is preferably coated with a thin layer of a precious metaloxide. The reason for this is as follows. A coating formed byplasma-spraying of nickel oxide has a thickness of 100 μm or more and ishard and brittle for the zero-gap electrode which requires flexibility,and an ion exchange membrane in contact with the cathode may easily bedamaged. Further, with a metal plating, a sufficient level of activityis hard to obtain. Therefore, the coating made mainly of a preciousmetal oxide is preferable since it is highly active and can make thecoating layer thin.

A small thickness of the coating layer is preferred as it keeps thecathode base material flexible and therefore protects the ion exchangemembrane from damage. If the coating is thicker, manufacturing cost isincreased and the coating may damage the ion exchange membrane. However,if the coating is too thin, it may not provide sufficient activity.Thus, a coating layer thickness is preferably from 0.5 μm to 50 μm, morepreferably in a range between 1 μm and 10 μm. The coating thickness ofthe cathode can be measured by cutting the base material and using anoptical microscope or electronic microscope.

Such a cathode can be mounted using a commonly known welding techniqueor pins.

In the zero-gap electrolytic cell, in addition to the requirementsdescribed above, the geometry of the anode itself is also important. Theion exchange membrane is pushed against the anode with more forcestronger than in the conventional finite gap electrolytic cell, and ifthe anode is made of an expanded metal base material, the ion exchangemembrane may be damaged at the end of an opening or it may cut into theopening so that a gap is formed between the cathode and the ion exchangemembrane and the voltage is increased.

The electrode therefore has to be formed as planar as possible. For thatpurpose, it is preferable to press the expanded base material with aroller and form it in a planar shape. Generally, the expanding processincreases the apparent thickness to about 1.5 to 2 times the thicknessbefore the processing. Using the expanded material as is for thezero-gap electrolytic cell causes the aforementioned problems and thusthe expanded material is preferably rolled by a roll press to beplanarized and to reduce its thickness to 95-105% of the thickness ofthe metal plate before the processing. This may prevent damage to theion exchange membrane and unexpectedly reduce the voltage. The reasonfor this is not entirely clear. However, it is believed that when thesurface of the ion exchange membrane and the electrode surface areuniform, there is intimate contact and the current density becomesuniform.

The thickness of the anode is preferably from 0.7 mm to 2.0 mm in anordinary case. Too small a thickness will cause the anode to sink by thepressure of the ion exchange membrane pushing the anode, which is causedby a pressure difference between the anode chamber and the cathodechamber and by the pressing force of the cathode. This widens theelectrode distance, increases the voltage of the zero-gap electrolyticcell and therefore is not desirable. On the other hand, too large athickness will cause an electrochemical reaction on the back of theelectrode, i.e., on the side opposite its surface in contact with theion exchange membrane, thus increasing the resistance and is notdesirable.

A more preferred thickness of the anode is between 0.9 mm and 1.5 mm andeven more preferably between 0.9 mm and 1.1 mm. In the case of the metalwire net, two wires cross each other at a right angle, and the thicknessis two times the wire diameter.

In the zero-gap electrolytic cell, the ion exchange membrane and theelectrode surface are in intimate contact during electrolysis, and thesupply of the electrolyte may locally become short. In the zero-gapelectrolytic cell, chlorine gas is produced on the anode side duringelectrolysis and hydrogen gas is provided on the cathode side. Normally,the electrolysis operation is performed by maintaining the gas pressureon the cathode side higher than the gas pressure on the anode side andpressing the membrane against the anode by the gas pressure difference.In the zero-gap electrolyzer, the pressing force is applied to the anodeside also from the mattress on the cathode side during the operation, sothat the pressure acting on the anode side is higher than the pressurein the finite gap electrolyzer that normally has a gap between the anodeand the cathode. As the pressing force becomes large, fine bubbles mayform in the ion exchange membrane or the electrolytic voltage mayincrease.

To prevent these problems, the anode preferably has irregularitiesformed in the anode surface such that electrolyte feeding is facilitatedthough the irregularities. More specifically, it is effective to formappropriate irregularities in the anode surface by blasting or acidetching.

The irregularities are applied with an anode catalyst, which fills therecesses and makes the surface less-rough than it was immediately afteretching. For example, the anode catalyst is formed by acid-treating thesurface of the titanium base material, applying to the surface a mixedsolution of iridium chloride, ruthenium chloride and titanium chloride,and then thermally decomposing the solution. By repeating the process ofapplying the catalyst to a thickness of 0.2-0.3 μm and thermallydecomposing the catalyst, a catalyst layer can be formed to a totalthickness of 1-10 μm on average. While the thickness of the catalystlayer is determined in view of the lifetime and the price of the anode,it is preferably selected in the range of between 1 μm and 3 μm onaverage.

As for the surface roughness after the anode catalyst application, it isrequired that a maximum difference between ridges and troughs on thesurface is in a range of between 5 μm and 50 μm. When the irregularitiesare too small, the supply of electrolyte may become insufficient incertain locations, and this is not desirable. When the irregularitiesare too large, the surface of the ion exchange membrane may be damaged,and this is undesirable as well. Therefore, for stable use of the ionexchange membrane, the maximum height difference between ridges andtroughs in the anode surface needs to fall in the range of between 5 μmand 50 μm. For more stable operation, it is further preferred that themaximum height difference of the irregularities on the anode surface bein the range from 8 μm to 30 μm.

Either a contact type measuring method using a probe or a non-contacttype measuring method using optical interference and laser light can beused to measure the surface roughness of the anode. After havingundergone the expanding process, rolling process, acid processing andcatalyst application, the anode will have fine irregularities in itssurface that cannot be detected with a probe. So, the non-contact typemeasuring method is preferred.

The measurement using the non-contact type optical interference methodmay use the NewView5022 scanner from Zygo or a different device. TheZygo device has an optical microscope and an interference type objectlens/CCD camera. The device three-dimensionally measures the surfacegeometry of a target and calculates the irregularities by irradiating awhite light against the target and vertically scanning interferencefringes that form according to the surface geometry.

Although the area to be measured can be selected arbitrarily, it ispreferable to measure an area of 10-300 μm² in order to properly knowthe irregularities of the anode surface. Particularly when measuring anexpanded metal, it is more preferable to measure an area of 50-150 μm².

While measurements, such as average surface roughness Ra, and the10-point average surface roughness, can be taken, a difference betweenthe maximum height value and the minimum height value of surfaceirregularities is calculated as a PV (peak-to-valley) value. Theinventors have found a significant correlation between the anode surfaceroughness as represented by the PV value and test results using thezero-gap electrolyzer, and have achieved the present invention. In thefollowing description, the PV value means the maximum height differenceof irregularities on the anode surface.

The opening percentage of the anode base material is preferably set in arange of 25-70%. There are a variety of methods for measuring theopening percentage. Measurement may be made by any of them, for instanceby a method of copying an electrode sample by a copying machine, cuttingout opening portions and calculating a weight or by a method ofmeasuring the length and width of opening portions and calculating thepercentage.

If the opening percentage is too small, the supply of electrolyte to theion exchange membrane may become insufficient, resulting in generationof bubbles, which in turn gives rise to a possibility that theelectrolyzer may not be operated with stable voltage and currentefficiency, and this is not desirable. If the opening percentage is toolarge, on the other hand, the surface area of the electrode decreasesand the voltage increases, which is undesirable. Thus, the mostpreferred opening percentage is in a range of 30-60%.

For performing electrolysis by using a zero-gap electrolytic cell,according to the study by the inventors, the most preferable methodincludes the use of the bipolar, zero-gap type electrolytic cell havingat least one cylindrical duct or baffle plate that forms an internalcirculation path for the electrolyte, between a separation wall of theanode chamber and/or cathode chamber and the electrode. This cell has atleast three layers on the cathode side, which are a conductive platelayer, a conductive cushion mat layer stacked on the conductive platelayer and a hydrogen generating cathode layer of a 0.5 mm or lessthickness, stacked on the cushion mat layer in an area where it contactsthe cationic exchange membrane. In such a zero-gap electrolytic cell,the electrolyte concentration distribution on the anode side and on thecathode side can be adjusted easily and properly. Further, in-cellpressure variations are small and the ion exchange membrane is almostfree from damages. Therefore stable electrolysis can be performed for along period of time even at a high current density of about 8 kA/m².

What is required to operate the zero-gap electrolyzer at stable currentefficiency and stable voltage with a high current density of 4-8 kA/m²,more preferably 5-8 kA/m², for a long period is that the electrolyteconcentration distribution in the cell is uniform, that no stagnantbubbles or gas stay in the cell, and that, when the electrolyte, bubblesand gas are discharged from a discharge nozzle, they do not form amixture of different phases. These provisions prevent pressurevariations and therefore vibrations from occurring in the cell.Vibrations in the cell are measured by using an AR1200 analyzingrecorder from Yokogawa Denki, which measures pressure variations in theanode cell and by taking a difference between the maximum pressure andthe minimum pressure as the vibrations in the electrolyzer.

In the zero-gap cell, the anode and the cathode are held in intimatecontact with each other through the ion exchange membrane. The movementof substance toward the ion exchange membrane can be easily obstructed.When the substance movement to the ion exchange membrane is impeded,various undesired influences, such as bubbles being formed in the ionexchange membrane, a voltage rise and a degraded current efficiencyoccur. It is therefore important to facilitate the substance movement tothe ion exchange membrane to keep the electrolyte concentrationdistribution in the cell uniform.

The study of the inventors has revealed that there is a correlationbetween the concentration distribution on the anode side and a tendencyfor the current efficiency of the ion exchange membrane to deteriorate,and that the reduction in current efficiency becomes large as theconcentration distribution widens. This tendency is particularly markedwhen the current density is high and when the gap is zero. Measurementswere made of the electrolyte concentration at nine sampling positions 13in the anode chamber as shown with black dots in FIG. 3, and theconcentration difference was obtained by subtracting the minimumconcentration from the maximum concentration. It has been found that, inthe current density range of 4-8 kA/m², the current efficiency reductionis significant when the concentration difference is greater than 0.5 N.Therefore, in the zero-gap electrolyzer, for the current density of 4-8kA/m² it is preferable to set the brine concentration difference to beless than 0.5 N.

Generally, the anode side of a chlor-alkali electrolyzer is greatlyaffected by bubbles. For example, under the electrolysis conditions of 4kA/m², 0.1 MPa and 90° C., an upper part of the anode chamber is filledwith bubbles and there are regions where a gas/liquid ratio is more than80%. In areas with such a high gas-liquid ratio, when the currentdensity increases, the electrolyte concentration distribution ordifference tends to widen. The areas of a high gas-liquid ratio have lowfluidity and therefore may cause locally a reduced electrolyteconcentration and stagnancy of gas. To reduce a space with a largegas-liquid ratio in the upper part of the electrode chamber, currentlyavailable are methods that increase the electrolytic pressure and thatgreatly increase the circulating volume of electrolyte. These methods,however, have safety problems and tend to increase the facility buildingcost and are not desirable. With a high current density of more than 4kA/m², the influence of bubbles significantly increases as the volume ofgas produced increases, and there are cases where some areas in the cellare insufficiently agitated, a salt water consumption in the anodechamber is accelerated, and the electrolyte concentration distributionin the electrolytic cell becomes nonuniform.

There are some measures available in the zero-gap cell to prevent suchelectrolyte concentration distribution deterioration in the anodechamber and not to impede substance movement to the ion exchangemembrane. For example, such an anode side construction as shown in FIG.3 and FIG. 4 is appropriate for the zero-gap cell, that has a plate forinternal circulation in the cell to allow uniform supply of electrolytein a lateral direction.

More specifically, as shown in FIG. 3 and FIG. 4, saturated salt watersupplied uniformly in the lateral direction through an anode liquiddistributor 14 is circulated vertically in the cell by a baffle plate 9to provide uniform electrolyte concentration distribution in the wholecell. Using this electrolytic cell, the electrolyte concentrationdistribution can be adjusted more precisely by collecting lean saltwater discharged from an outlet nozzle 8 and mixing it with thesaturated salt water to increase the volume of salt water and lower itsconcentration for re-supply. This enables the zero-gap electrolytic cellto perform electrolysis with a stable performance.

The electrolyte concentration distribution on the cathode sidecorrelates with a tendency for the ion exchange membrane voltage torise. It has been found that the voltage increase becomes large as theelectrolyte concentration distribution or difference widens. For a highcurrent density, this tendency becomes significant particularly when thegap is zero. Also in the cathode chamber, as shown in FIG. 8, theelectrolyte concentration was measured at nine sampling positions 13, asin the case with the anode chamber, and a concentration differenceobtained by subtracting the minimum concentration from the maximumconcentration. It was found that, in the current density range of 4-8kA/m², the current efficiency decreased significantly when theconcentration difference was greater than 2%. Therefore, in the zero-gapelectrolyzer, for the current density of 4-8 kA/m² it is preferable toset the alkaline concentration difference to be less than 2%.

There are some measures available in the zero-gap cell to preventdeterioration of the electrolyte concentration distribution in thecathode chamber and to not impede substance movement near the ionexchange membrane. For example, such a cathode side construction asshown in FIG. 6 and FIG. 8 is an appropriate construction for thezero-gap cell, which allows uniform supply of electrolyte in a lateraldirection.

More specifically, as shown in FIG. 8, the electrolyte supplieduniformly in the lateral direction through a cathode liquid distributor23 is circulated vertically in the cell according to a concentrationdifference between the alkali supplied and the alkali in the cathodechamber in order to provide uniform electrolyte concentrationdistribution in the whole cell. Using this electrolytic cell, theelectrolyte density distribution can be adjusted more precisely byproperly adjusting the alkali flow being supplied. This enables thezero-gap electrolytic cell to perform electrolysis at a stable voltage.

When a pressure variation occurs in the electrolytic cell, a pressuredifference between the anode chamber and the cathode chamber varies. Inthe zero-gap electrolytic cell, the cushion mat is used to keep theanode and the cathode in intimate contact with each other through theion exchange membrane at all times. If the pressure difference varies,the force for the intimate contact also varies, with the result that theion exchange membrane may be rubbed by the electrodes. The ion exchangemembrane is made of resin and its surfaces are coated to prevent theadhesion of gas, so if the ion exchange membrane is rubbed by theelectrodes, the coating layer on the ion exchange membrane may bescraped off or the ion exchange membrane itself may be chipped off. Inthat case, a voltage increase and deterioration of current efficiencywill result, thus making stable electrolysis impossible. Therefore,preventing a pressure variation in the electrolytic cell is an importantfactor for the zero-gap electrolytic cell. Such a pressure variation inthe cell is preferably kept as small as possible, e.g., to less than 30cm-H₂O or more preferably to less than 15 cm-H₂O, or most preferably toless than 10 cm-H₂O. If the pressure variation is smaller than 10cm-H₂O, the ion exchange membrane will have no damage and can be put incontinued operation even after a long-term electrolysis operation ofmore than one year.

Some measures are available to prevent pressure variations in the cell.For example, as shown in FIG. 5, it is effective to provide a partitionplate 20 in a gas-liquid separation chamber 7 and also a bubble removingporous plate 19 on the top of the partition plate 20.

Embodiments of the invention and their applications will now bedescribed. The present invention, however, is not limited to thesespecific forms.

APPLICATION EXAMPLE 1

The bipolar, zero-gap type electrolytic cells 30 according to anembodiment of the invention, each of which has an anode structure and acathode structure similar to those of FIG. 3 and FIG. 8 and across-sectional structure similar to that shown in FIG. 6, are arrangedin series and assembled into an electrolyzer as shown in FIG. 7. FIG. 7shows an anode unit cell disposed at one end of the assembly and acathode unit cell disposed at the other end and with current lead plates28 attached as shown.

The bipolar, zero-gap type electrolytic cell 30 measures 2400 mm wide by1280 mm high and has an anode chamber, a cathode chamber and agas-liquid separation chamber 7. The anode chamber and the cathodechamber are each formed by a flat pan-shaped separation wall 5 and arearranged back to back. These anode chamber and cathode chamber arecombined together by inserting a frame member 22 into a bent portion 18provided at the top of the separation wall 5. Each gas-liquid separationchamber is defined in the upper part of each electrode chamber by fixingan L-shaped partition member 16 of a height H to the separation wall 5.

The gas-liquid separation chamber on the anode side has across-sectional area of 27 cm², on the cathode side has across-sectional area of 15 cm², and only the gas-liquid separationchamber on the anode side has a similar construction to that shown inFIG. 5. That is, in the gas-liquid separation chamber on the anode sideis installed a titanium partition plate 20 having a height H′ of 50 mmand a thickness of 1 mm, with a width W of a passage B set to 5 mm. Onthe top of the partition plate a titanium expanded metal porous plate 19having an opening percentage of 59% and a thickness of 1 mm is mountedwith a height rising vertically up to the upper end of the gas-liquidseparation chamber. Holes 15 in the anode side gas-liquid separationchamber are in an elliptical shape 5 mm wide and 22 mm long and arearranged at a 37.5-mm pitch.

The baffle plate 9 is provided only on the anode side. A titanium baffleplate with a thickness of 1 mm and a height H2 of 500 mm is installed,with a width W2 of a passage D set to 10 mm and a gap W2′ between theseparation wall 5 and the lower end of the plate set to 3 mm. A verticaldistance S from the upper end of the baffle plate to the upper end ofthe electrode chamber is set to 40 mm.

The anode liquid distributor 14 comprises a square pipe having a lengthof 220 cm and a cross-sectional area of 4 cm², which is formed with 24holes at equal intervals, each measuring 1.5 mm across, and which isinstalled horizontally at a position 50 mm above the bottom of the anodechamber of the cell, with one end joined to an anode side inlet nozzle12. A pressure loss of this distributor was about 2 mm-H₂O whensaturated salt water of 150 L/Hr equivalent to 4 kA/m² was supplied.

A cathode liquid distributor 23 comprises a square pipe having a lengthof 220 cm and a cross-sectional area of 3.5 cm², which is formed with 24holes at equal intervals, each measuring 2 mm across, and which ismounted horizontally at a position 50 mm above the bottom of the cathodechamber of the cell, with one end joined to a cathode side inlet nozzle.A pressure loss of this distributor was about 12 mm-H₂O when alkali of300 L/Hr equivalent to 4 kA/m² was supplied.

As a zero-gap structure on the cathode side, a structure shown in FIG. 1was manufactured. More specifically, the conductive plate 3 is a nickelexpanded metal having a thickness of 1.2 mm thick, with openings eachmeasuring 8 mm in lateral length and 5 mm in longitudinal length. Thecushion mat 2 has four nickel wires of a 0.1 mm diameter, which arewoven into a mat and then corrugated to a thickness of 9 mm. This mat issecured to the conductive plate 3 by spot-welding at 18 locations. Themat is then covered with a 40 mesh nickel wire net of a 0.15-mm wirediameter, which is coated with a material mainly composed of rutheniumoxide to a thickness of about 3 μm and forms the hydrogen generatingcathode 1. The hydrogen generating cathode 1 is secured to theconductive plate 3 by spot-welding at about 60 locations along theperiphery of the cathode. The cathode side zero-gap structure is thusconstructed of three layers.

The anode side structure has the anode liquid distributor 14 as shown inFIG. 3 and the baffle plate 9 as shown in FIG. 3 and FIG. 4.

To prevent pressure variations in the cell, the partition plate 20 andthe bubble eliminating porous plate 19, shown in FIG. 5, are provided inthe anode side gas-liquid separation chamber. They are not provided inthe cathode side gas-liquid separation chamber.

The anode 11 is a titanium plate of a 1 mm thickness, which is expanded,roll-pressed to a thickness of 1±0.05 mm and secured to ribs 22. Theopening portions of the expanded metal before being roll-pressed are ata pitch of 6 mm in horizontal direction and 3 mm in longitudinaldirection with a machining pitch is set to 1 mm. The opening percentageof the expanded metal after roll-pressing was measured by a copyingmachine and found to be 40%. The expanded metal was etched with sulfuricacid, and the maximum height difference between the ridges and troughs(the irregularities) on the surface was 30 μm. The base material isetched with acid and then coated with a material mainly composed ofRuO₂, IrO₂ and TiO₂ to form the anode. The maximum height differencebetween the ridges and troughs (the irregularities) on the anode surfaceafter the coating was about 13 μm.

The maximum height difference between the irregularities on the anodesurface was measured by using the NewView5022 scanner from Zygo.

First, a calibration was performed using a standard sample where theirregularities were set to 1.824 μm so that an appropriate amount oflight could be obtained. Then, a target object was put under a whitelight source and an adjustment was made to cause interference fringes toappear. Then, a measurement was taken of the interference fringes as theobject was moved about 100 μm vertically, the irregularities weredetermined by a frequency area analysis, and a difference between themaximum and minimum values was calculated to be a maximum differencebetween the ridges and the troughs (the irregularities).

A cationic exchange membrane ACIPLEX® F4401 was sandwiched between theelectrolytic cells of the above construction through gaskets to form theelectrolyzer. Salt water with a concentration of 300 g/L was supplied asan anode liquid to the anode chamber side of this electrolyzer so thatan outlet salt water concentration would be 200 g/L. Lean caustic sodawas supplied to the cathode chamber side so that an outlet caustic sodaconcentration would be 32% by weight. An electrolysis operation wasperformed for 360 days at an electrolysis temperature of 90° C., anabsolute pressure of 0.14 MPa during electrolysis and a current densityof 4-6 kA/m².

The anode liquid concentration distribution and the cathode liquiddensity distribution in the electrolytic cell during the electrolysisoperation were measured at the sampling points 13 shown in FIG. 3 andFIG. 8. More specifically, the measurement was taken at nine pointswhich were 150 mm, 600 mm and 1000 mm below the top of the conductingportion in the cell and at the center of the cell and 100 mm inside fromthe both ends of the cell. Differences between the maximum and minimumconcentrations at the nine points are shown as concentration differencein Table 1. TABLE 1 Application example 1 5 kA/m² 6 kA/m² First 30300-360 First 30 300-360 days days days days Average voltage (V) 2.902.92 2.99 3.02 Voltage change (mV) 20 30 Average current 96.7 96.0 96.595.5 efficiency (%) Current efficiency 0.7 1.0 change (%) Volume of saltwater 193 232 supplied (L/Hr-cell) Volume of lean salt 25 25 waterrecycled (L/Hr-cell) (L/Hr-cell) In-cell salt water 0.31 0.35concentration difference (N) NaOH supply volume 300 300 (L/Hr-cell)Concentration of 30.4 30.6 supplied NaOH (%) In-cell NaOH 0.6 0.8concentration difference (%) Anode side in-cell Less than 5 Less than 5pressure variation (cm-H₂O) (cm-H₂O) State of ion exchange No pin holesor bubbles were membranes after 360 days observed on ion exchangemembranes.

Further, Table 1 shows measurement of the average voltage and voltagechange, current efficiency, and vibrations and concentrationdistribution in the cells during the electrolysis operation. Table 1shows that a voltage rise was as small as 30 mV for 6 kA/m² and thatcurrent efficiency degradation was also as small as 1%. Vibrations inthe electrolytic cell were less than 5 cm in the water column and theconcentration difference was 0.31-0.35 N on the anode side and 0.6-0.8%on the cathode side.

After 360 days of the electrolysis operation, the electrolyzer wasdisassembled to take out the ion exchange membranes for examination. Theion exchange membranes had no bubbles and were in a good state forfuture use and operation.

COMPARISON EXAMPLE 1

An electrolyzer was manufactured by using similar bipolar electrolyticcells except that the anodes used in the application example 1 weremodified.

More specifically, the titanium plate of a 1 mm thickness of the anodewas expanded to have an opening percentage of 30% and then etched withsulfuric acid to form irregularities on its surface whose maximum heightdifference was about 8 μm. The expanded titanium plate was then coatedwith a material composed mainly of RuO₂, IrO₂ and TiO₂. The maximumheight difference between the irregularities on the coated surface was 3μm and the thickness of the anode was 1.8 mm. This electrolyzer wasoperated under exactly the same conditions as application example 1 anda similar measurement was made. Measured values are shown in Table 2.Table 2 shows that a voltage rise was as high as 150 mV for 6 kA/m² andcurrent efficiency reduction was as large as 2-3%. Vibrations in theelectrolytic cell were less than 5 cm in the water column for 6 kA/m²and a concentration difference was 0.31-0.35 N on the anode side and0.6-0.8% on the cathode side.

After the 360 days operation, the electrolyzer was disassembled to takeout the ion exchange membranes for examination. The ion exchangemembranes were found to have fine bubbles and some were formed withsmall pin holes.

REFERENCE EXAMPLE 1

An electrolyzer was built by using similar bipolar electrolytic cellsexcept that the hydrogen generating cathodes used in application example1 were modified. Used as the hydrogen generating cathode was a 14 meshnickel wire net of a 0.4 mm wire diameter (a cathode thickness of 0.8mm) coated with a material composed mainly of nickel oxide to athickness of about 250 μm.

After the electrolyzer was operated under exactly the same conditions asthe application example 1, similar measurements were made. The resultsare shown in Table 2. The results show that voltage was relatively highfrom the initial stage, that its rise was as large as 80 mV for 6 kA/m²and that the current efficiency degradation was as great as 2-3%.Vibrations in the electrolytic cell were less than 5 cm in the watercolumn for 6 kA/m² and a concentration difference was 0.31-0.35 N on theanode side and 0.6-0.8% on the cathode side.

After 360 days of the operation, the electrolyzer was disassembled totake out the ion exchange membranes for examination. The surface of theion exchange membranes were scraped off. Some were formed with small pinholes. The cathode coating was heavily scraped and cracked. TABLE 2Reference Comparison example 1 example 1 5 kA/m² 6 kA/m² 6 kA/m² FirstFirst First 30 300-360 30 300-360 30 300-360 days days days days daysdays Average 2.95 3.08 3.05 3.20 3.04 3.12 voltage (V) Voltage 130 15080 change (mV) Average 96.3 93.8 96.1 93.5 96.1 93.3 current efficiency(%) Current 2.5 2.6 2.8 efficiency change (%) Volume of 193 232 232 saltwater supplied (L/Hr-cell) Volume of 25 25 25 lean salt (L/Hr-cell)(L/Hr-cell) (L/Hr-cell) water recycled In-cell salt 0.31 0.35 0.35 waterconcentration difference (N) NaOH supply 300 300 300 volume (L/Hr-cell)Concentra- 30.5 30.5 30.5 tion of supplied NaOH (%) In-cell NaOH 0.6 0.80.8 concentration difference (%) Anode side Less than 5 Less than 5 Lessthan 5 in-cell (cm-H₂O) (cm-H₂O) (cm-H₂O) pressure variation State ofion Almost all ion exchange Many ion exchange membranes were found withexchange membranes bubbles; some had pin holes membrane after 360surfaces were days damaged and pin-holed.

APPLICATION EXAMPLE 2

An electrolyzer was built by using similar bipolar electrolytic cellsexcept that the anodes used in application example 1 were modified.

A titanium plate of 1 mm thickness was used as the anode and thetitanium plate was expanded and roll-pressed to a thickness of 1.2 mm.An opening percentage was measured to be 40%. The expanded titaniumplate was etched with sulfuric acid to form irregularities on itssurface whose maximum height difference was about 30 μm. It was thencoated with a material composed mainly of RuO₂, IrO₂ and TiO₂. Themaximum height difference between the irregularities on the coatedsurface was 13 μm. The electrolyzer was operated under exactly the sameconditions as application example 1 and a similar measurement was made.Measured values are shown in Table 3. Table 3 shows that a voltage risewas 50 mV for 6 kA/m² and current efficiency degradation was 1.3%.Vibrations in the electrolytic cell were less than 5 cm in the watercolumn for 6 kA/m² and a concentration difference was 0.31-0.36 N on theanode side and 0.6-0.8% on the cathode side.

After 360 days of the electrolysis operation, the electrolyzer wasdisassembled to take out the ion exchange membranes for examination. Theion exchange membranes had no bubbles and were in a good state forfuture use and operation. TABLE 3 Application example 2 5 kA/m² 6 kA/m²First 30 300-360 First 30 300-360 days days days days Average voltage(V) 2.93 2.96 3.02 3.07 Voltage change (mV) 30 50 Average current 96.795.8 96.5 95.2 efficiency (%) Current efficiency 0.9 1.3 change (%)Volume of salt water 193 232 supplied (L/Hr-cell) Volume of lean salt 2525 water recycled (L/Hr-cell) (L/Hr-cell) In-cell salt water 0.31 0.36concentration difference (N) NaOH supply volume 300 300 (L/Hr-cell)Concentration of 30.4 30.6 supplied NaOH (%) In-cell NaOH 0.6 0.8concentration difference (%) Anode side in-cell Less than 5 Less than 5pressure variation (cm-H₂O) (cm-H₂O) State of ion exchange No pin holesor bubbles were membranes after 360 days observed on ion exchangemembranes.

APPLICATION EXAMPLE 3

Electrolysis was performed in a range of 7-8 kA/m² using the sameelectrolyzer as in application example 1.

In this operation, lean brine discharged when the anode liquid from theelectrolyzer was added in a maximum volume of 155 L/Hr-cell to thesaturated salt water and supplied each electrolytic cell a desiredconcentration distribution. For the cathode liquid also, a supply volumewas changed up to 400 L/Hr-cell to keep desired concentrationdistribution.

Voltage, current efficiency, and vibrations and concentrationdistribution in the cells during the electrolysis operation weremeasured. The results are shown in Table 4. Table 4 shows that a voltagerise was as small as 30 mV for 8 kA/m² and that the current efficiencydegradation was as small as 0.9%. Vibrations in the cell were less than10 cm in water column and a concentration difference was 0.39-0.47 N onthe anode side and 1.2-1.4% on the cathode side.

After 180 days of the electrolysis operation, the electrolyzer wasdisassembled to take out the ion exchange membranes for examination. Theion exchange membranes had no bubbles and were in a good state forfuture use and operation.

REFERENCE EXAMPLE 2

Electrolysis was performed in a range of 7-8 kA/m² using exactly thesame electrolyzer as application example 1.

The electrolysis was conducted under similar conditions to those ofapplication example 3, except that the lean brine discharged from theelectrolyzer as the anode liquid was not added to the saturated brineand the supply volume of cathode liquid was kept at 300 L/Hr-cell.

Voltage, current efficiency, and vibrations and concentrationdistribution in the cells during the electrolysis operation weremeasured and the results are shown in Table 4. The result shows that avoltage rise was 90 mV for 8 kA/m² and that the current efficiencydegradation was 3.3%. Vibrations in the cell were less than 5 cm inwater column and a concentration difference was 0.6-0.7 N on the anodeside and 1.5-2.1% on the cathode side.

After 180 days of the electrolysis operation, the electrolyzer wasdisassembled to take out the ion exchange membranes for examination. Theion exchange membranes had many bubbles measuring 1-10 mm in diameteradhering to their entire surfaces. TABLE 4 Application example 3Reference example 2 7 kA/m² 8 kA/m² 7 kA/m² 8 kA/m² First 150- First150- First 150- First 150- 30 180 30 180 30 180 30 180 days days daysdays days days days days Average 3.09 3.11 3.18 3.21 3.08 3.16 3.17 3.26voltage (V) Voltage 20 30 80 90 change (mV) Average 96.3 95.5 96.1 95.296.1 92.9 96.0 92.7 current efficiency (%) Current 0.8 0.9 3.2 3.3efficiency change (%) Volume of 337 465 270 310 salt water supplied(L/Hr-cell) Volume of 67 155 25 25 lean salt (L/Hr-cell) (L/Hr-cell)(L/Hr-cell) (L/Hr-cell) water recycled In-cell 0.39 0.47 0.61 0.73 saltwater concentra- tion difference (N) NaOH supply 350 400 300 300 volume(L/Hr-cell) Concentra- 30.5 30.5 30.5 30.5 tion of supplied NaOH (%)In-cell 1.2 1.4 1.5 2.1 NaOH concentra- tion difference (%) Anode side 88 Less than 5 Less than 5 in-cell (cm-H₂O) (cm-H₂O) (cm-H₂O) (cm-H₂O)pressure variation State No abnormalities Many bubbles of ion were foundin ion were observed. exchange exchange membranes. membranes after 360days

APPLICATION EXAMPLE 4

A bipolar electrolytic cell was prepared with a cross-sectionalstructure as shown in FIG. 9, an expanded metal of a 1.8 mm thickness asthe anode and a nickel expanded metal as the cathode. The cathode iscoated with a material composed mainly of nickel oxide by plasmaspraying to a thickness of 250 μm. The electrolytic cell was used forone year with the electrode distance set to 2 mm.

The anode of this cell was taken out and a new anode with the exactconfiguration of application example 1 was installed in its place.Further, the coating on the cathode was scraped off by a brush to exposea nickel base metal to be used as a conductive plate. The same cushionmat and hydrogen generating cathode as those of application example 1were mounted in exactly the same way.

An electrolyzer similar to that of application example 1 was built.Electrolysis was then performed in a similar manner. Voltage, currentefficiency, and vibrations and the concentration distribution in thecells during the electrolysis operation were measured. The results areshown in Table 5. Table 5 shows that a voltage rise was only 20 mV for 6kA/m² and that the current efficiency degradation was as small as 0.7%.Vibrations in the cell were less than 5 cm in water column and aconcentration difference was 0.35 N at maximum on the anode side and0.8% at maximum on the cathode side.

After 180 days of the electrolysis operation, the electrolyzer wasdisassembled to take out the ion exchange membranes for examination. Theion exchange membranes had no bubbles and were in a good state good forfuture use and operation. TABLE 5 Application example 4 5 kA/m² 6 kA/m²First 30 150-180 First 30 150-180 days days days days Average voltage(V) 2.91 2.92 3.00 3.02 Voltage change (mV) 10 20 Average current 96.896.2 96.6 95.9 efficiency (%) Current efficiency 0.6 0.7 change (%)Volume of salt water 193 232 supplied (L/Hr-cell) Volume of lean salt 2525 water recycled (L/Hr-cell) (L/Hr-cell) In-cell salt water 0.32 0.35concentration difference (N) NaOH supply volume 300 300 (L/Hr-cell)Concentration of 30.5 30.5 supplied NaOH (%) In-cell NaOH 0.6 0.8concentration difference (%) Anode side in-cell Less than 5 Less than 5pressure variation (cm-H₂O) (cm-H₂O) State of ion exchange No pin holesor bubbles membranes after 360 days were observed on ion exchangemembranes.

INDUSTRIAL APPLICABILITY

The bipolar, zero-gap type electrolytic cell has the gas-liquidseparation chambers 7 in non-conducting portions in upper parts of theanode and cathode chambers, each of which is formed integrally with theanode chamber or the cathode chamber, at least one cylindrical duct orbaffle plate 9 is installed between a separation wall 5 of the anodechamber and/or cathode chamber and the electrodes to form an internalcirculation path for the electrolyte, and three-layers on the cathodeside, which comprise a conductive plate 3, a conductive cushion mat 2stacked on the conductive plate, and a hydrogen generating cathode 1placed on the cushion mat in an area where it contacts a cationicexchange membrane. In this bipolar, zero-gap type electrolytic cell,since the anode is optimally shaped, performing electrolysis at acurrent density of 4-8 kA/m² does not cause the voltage to rise as timeelapses. Only a small reduction in the current efficiency occurs and itproduces bubbles in the ion exchange membranes. With this, stableelectrolysis can be performed for a long period of time.

Such a zero-gap electrolytic cell can also be manufactured by modifyingthose electrolytic cells using a finite gap structure. This modificationof the finite gap cell into a zero-gap cell can be done for thoseelectrolytic cells that have been used as finite gap cells and comprisegas-liquid separation chambers formed in non-conducting portions inupper parts of the anode and cathode chambers. The electrolytic cellsform within the anode chamber or the cathode chamber. A cylindrical ductor baffle plate is installed between a separation wall of the anodechamber and/or cathode chamber in order for the electrodes to form aninternal circulation path for the electrolyte. In this case, the anodeand the anode chamber are modified into the structure described above,and then the cathode chamber is also modified. A conductive plate, acushion mat and a cathode are then installed to form a zero-gap cellstructure. A zero-gap electrolytic cell can also be manufactured simplyby using the cathode that has been used in the finite gap cell as theconductive plate. Then, a cushion mat and a cathode are newly stacked onthe conductive plate. Conversely, the zero-gap cell can be used as afinite gap cell by removing the cathode, the cushion mat and theconductive plate from the zero-gap cell and then by installing a newcathode. This modification is less expensive than manufacturing a newelectrolytic cell and can be implemented easily, so it offers a greatadvantage for the user.

1. A bipolar, zero-gap type electrolytic cell for use in a filter presstype electrolyzer having a plurality of bipolar electrolytic cells and aplurality of anodic ion exchange membranes each arranged betweenadjacent bipolar electrolytic cells, comprising: an anode chamber; ananode provided in the anode chamber, said anode being formed of an anodebase material comprising one of a titanium expanded metal and a titaniumwire netting with an open area percentage of 25% to 75%, said anode,after being applied with a catalyst on the anode base material, having amaximum height difference of 5 μm to 50 μm between irregularities on asurface thereof and a thickness of 0.7 mm to 2.0 mm; a cathode chamberarranged back to back with the anode chamber; and a cathode having atleast two layers stacked in the cathode chamber, said layers including aconductive cushion mat layer and a layer of a hydrogen generatingcathode, said hydrogen generating cathode layer being adjacent to thecushion mat layer and arranged in an area for contact with the anodicion exchange membrane.
 2. A bipolar, zero-gap type electrolytic cellaccording to claim 1, wherein said anode base material comprises thetitanium expanded metal that is formed during the expansion and rollingprocess of a titanium plate.
 3. A bipolar, zero-gap type electrolyticcell according to claim 2, wherein the metal has a thickness after theexpansion and rolling process of 95% to 105% of a plate thickness beforethe expansion process.
 4. A bipolar, zero-gap type electrolytic cellaccording to any one of claims 1 to 3, wherein said hydrogen generatingcathode is formed of a base material having a thickness of 0.05 mm to0.5 mm and is selected from a group of a nickel wire netting, anexpanded nickel metal and a punched, porous nickel plate, and saidhydrogen generating cathode has an electrolytic catalyst coating layerformed thereon that has a thickness of 50 μm or less.
 5. A bipolar,zero-gap type electrolytic cell according to claim 1, further comprisinggas-liquid separation chambers, said gas-liquid separation chambersbeing respectively formed in non-current-carrying parts at the top ofthe anode and cathode chambers within the anode and cathode chambers,wherein at least one of a cylindrical duct and a baffle plate serve asan inner circulation path for an electrolyte that is provided between atleast one partition wall portion of the anode and cathode chambers whichform the associated electrode.
 6. A bipolar, zero-gap type electrolyticcell according to claim 5, wherein said gas-liquid separation chambersare formed with partition plates therein.